This invention relates to micro-mirrors, and is particularly concerned with micro-mirrors having variable focal length and useful in optical components. The term xe2x80x9cmicro-mirrorxe2x80x9d is used herein to mean a mirror of relatively small size, typically having a lateral dimension of the order of 1 or 2 mm or less.
Optical components, for example for use in optical communications systems, typically couple an optical signal from an optical source to an optical sink; each of the optical source and sink can be an optical device such as an optical signal transmitter or detector, or an optical fiber or other optical path. It is desirable to perform such coupling with a minimal or, in the case of an optical attenuator, a controlled optical signal loss, which typically requires a matching between the sizes of the optical signal beam and the optical source and sink. Such matching can be difficult to achieve in practice, in view of constraints and tolerances of manufacture and temperature variations in use of the optical components.
For example, in the case of an optical receiver in which an optical signal is to be coupled from an optical fiber to an optical detector, it is known that the optical signal diverges from the output end of the optical fiber (the total beam angle is 2 sinxe2x88x921 (NA) where NA is the numerical aperture of the fiber; for example NA=0.14 giving a total beam angle of about 16xc2x0), so that the optical signal beam width increases with increasing distance from the fiber. It may not be practical to position an optical detector of a given size close enough to the end of the optical fiber to match the beam width; the detector instead may have to be positioned at a greater distance so that it intercepts only a part of the optical signal beam. Thus there is an undesirable mismatch between the detector and the optical signal beam.
It would be desirable to reduce this mismatch, and generally to provide optimal and/or controlled coupling of optical signals in optical components.
In the different field of microactuators, it is known to provide an electrothermal bimorph (a generalization of a bimetal) microactuator to provide relatively large deflection and actuating force due to a change in temperature. An article by Rashidian et al., xe2x80x9cElectrothermal Microactuators Based On Dielectric Loss Heatingxe2x80x9d, Proceedings of IEEE MEMS (Micro Electro Mechanical System), pages 24-29, Feb. 1993, discloses such microactuators in which the temperature change is due to heating as a result of dielectric loss due to an applied high frequency alternating (ac) voltage.
In the article by Rashidian et al., the microactuator has the form of a cantilevered beam or a bridge, comprising a polymer layer acting as a dielectric heater between two metal layers to which the alternating voltage is applied, with another underlying polymer layer. The article treats the two polymer layers as forming a bimorph due to different coefficients of thermal expansion.
The article by Rashidian et al. refers to the cantilevered beam or bridge having lengths from tens of microns (micrometers or xcexcm) to a few millimeters (mm), with specific reference to structures which are in one case 5 mm long and 300 xcexcm wide, and in another case 3 mm long and 240 xcexcm wide. The article does not otherwise discuss the width of the cantilevered beam or bridge. It can be appreciated that these microactuators are essentially linear or one-dimensional structures which are supported (by a silicon substrate) only at one end for a cantilevered beam, and only at the two ends for a bridge.
According to one aspect of this invention there is provided a micro-mirror comprising a membrane supported around its periphery, the membrane comprising first and second metal layers, a first polymer layer forming a dielectric between the first and second metal layers, and a second polymer layer adjacent to the second metal layer with the second metal layer being between the first and second polymer layers, the second metal layer and the second polymer layer having different coefficients of thermal expansion, whereby a high frequency alternating voltage applied between the first and second metal layers produces dielectric loss in the first polymer layer to heat and consequently curve the membrane, at least one of the first and second metal layers thereby forming a curved mirror with a focal length dependent upon said dielectric loss.
In contrast to a microactuator in the form of a one-dimensional cantilevered beam or bridge supported only at one or both ends as described above, the membrane of a micro-mirror in accordance with this aspect of the invention is supported around its periphery, so that it curves in two planes which are perpendicular to one another and to an undisplaced plane of the membrane. In addition, in use of the micro-mirror the displacement of the membrane due to heating does not serve to provide any actuating force, but instead at least one of the metal layers of the membrane acts as a curved mirror to reflect an optical signal beam.
The membrane, which is conveniently square but can alternatively be any other desired shape, can be supported substantially continuously or intermittently around its periphery, conveniently by a substrate with the periphery of the second polymer layer of the membrane adjacent to the substrate.
Such a micro-mirror can be of various sizes, but preferably has a free length of the membrane between its support around its periphery in a range from about 400 xcexcm to about 1800 xcexcm.
The micro-mirror can comprise various materials in layers of various thicknesses, but preferably at least one of the first and second metal layers comprises aluminum with a thickness of about 1.5 xcexcm or less, and at least one of the first and second polymer layers comprises polyvinylidene fluoride. For example, the first polymer layer can comprise a copolymer of polyvinylidene fluoride with a thickness of about 2.5 xcexcm or less, and the second polymer layer can comprise a homopolymer of polyvinylidene fluoride with a thickness less than about 10 xcexcm.
Another aspect of the invention provides, in combination, a micro-mirror as recited above and a high frequency alternating voltage source coupled to the first and second metal layers to determine a focal length of the micro-mirror.
A further aspect of the invention provides an optical component comprising a source and sink for an optical signal beam and a micro-mirror as recited above in an optical path between the source and sink for the optical signal beam.
In particular, at least one of the source and sink for the optical signal beam can comprise an optical fiber.
According to another aspect, this invention provides an optical component comprising a source and sink for an optical signal beam and a micro-mirror in an optical path between the source and sink for the optical signal beam, wherein the micro-mirror comprises a membrane arranged to reflect the optical signal beam, the membrane comprising first and second layers of electrically conductive material with a layer of a dielectric material between them, and a further layer, adjacent to the second layer of electrically conductive material, of a material having a different coefficient of thermal expansion from that of the second layer of electrically conductive material, the second layer of electrically conductive material being between said layer of dielectric material and said further layer, the membrane being supported and being such that a high frequency alternating voltage applied between the first and second layers of electrically conductive material produces heating of the layer of dielectric material to heat and consequently curve the membrane.
Preferably the first and second layers of electrically conductive material comprise metal layers at least one of which constitutes a reflective surface of the membrane, and the layer of dielectric material and said further layer comprise polymer layers.
In the above cases, where the optical component is an optical receiver the sink for the optical signal beam comprises an optical signal detector. The invention further extends to a combination of such an optical component, a high frequency alternating voltage source coupled to the first and second metal layers, and a feedback control path from an electrical output of the detector to the high frequency alternating voltage source to control a focal length of the micro-mirror.